Download No Slide Title - Earth and Environmental Sciences, Lehigh University

Geodynamics of Indentor Corners
Insight into the lithospheric structure and
deformation in Eastern
Tibet from splitting and traveltime
variations of core phases
S. Sol, A. Meltzer, B. Zurek, X. Zhang, J. Zhang
1Lehigh
University, Bethlehem, PA
2Institute of Geology & Min. Res., Chengdu, China
Map of the Eastern part of the Tibet plateau
Study area
LOCATION OF THE SEISMIC ARRAY
Shear-wave splitting or birefringence
As a shear (S) wave propagates through an anisotropic medium, it will split into
two roughly perpendicularly polarized shear waves and its components will
propagate at different speeds, resulting in a differential arrival time (split time, t)
between the components (shear-wave splitting or birefringence).
{
Fast polarization (F)
Delay time
Approximate expression for small
anisotropy
t=L*
Objectives: Assess the presence of seismic anisotropy and illuminate
important questions about the processes of deformation in the Earth
Where do we have evidence of anisotropy within the Earth?
Causes of anisotropy?
CRUST
- Thin isotropic layers
with different velocities
- Aligned, fluid fractures or
cracks
- LPO of lower crustal
phyllosilicates minerals
MANTLE
- LPO of mantle minerals
(Olivine)
Lattice-preferred orientations of olivine
Pole figures of crystallographic orientation of deformed olivine aggregates
- A-axis parallel to ductile shear
- S-wave polarization parallel to a-axis
The splitting technique can provide important information
about mantle strain field.
Paths of seismic phases used for the splitting analysis
The bulk of the evidence for mantle seismic anisotropy comes from
observations of SKS and SKKS phases.
Seismograms showing the arrival of different phases
Local event
SKS
Seismograms filtered appropriately for SKS phases
Example of SKS splitting analysis
How do you better constrain the depth of anisotropy
of the observable splitting?
From Savage, 1999
We can also use other geophysical methods!
Physical processes for producing mantle anisotropy
From Silver, 1996.
Topographic map displaying splitting results from two distinct azimuths
Pacific events
Bolivia event
Topographic map showing the preliminary splitting parameters
at all the stations of the Eastern Syntaxis Experiment
Comparing preliminary GPS results and splitting direction
GPS measurements are courtesy of Liu Yuping and Peter Koons
Complex anisotropic structure at the LHASA station (LSA)
F=50
F=30-40
F=25
F=-7 F=-30
F=60
Preliminary SKS relative traveltime analysis